1. Field of the Invention
The present invention relates to an antifriction bearing, particularly a rolling contact bearing such as, for example, that used with an inverter-controlled motor and a mounting adjacent to the motor, of a type used in an environment where electric corrosion is a primary problem. More specifically, the present invention relates to the rolling contact bearing of a type suited for use in an environment where problems associated with not only electric corrosion, but creepage of a bearing outer race tend to arise and which can advantageously used as a bearing for use on an free side of a shaft for permitting an axial displacement under the influence of a thermal expansion of the shaft.
2. Description of the Prior Art
Most electric motors currently largely utlized generally make use of a sealed, deep groove ball bearing. However, the use of invertor-controlled electric motor is a recent trend, and the increased use of the invertor-controlled electric motors has broght about a problem associated with electric corrosion occurring between rolling elements and one of the inner and outer races. The motors of the type referred to above are also susceptible to a problem associated with frictional wear occurring at an inner surface of a journal box, that encloses the bearing, as a result of a creepage in which an outer race of the bearing undergoes an circumferential displacement relative to the journal box receiving the outer race therein.
To minimize the electric corrosion, attempts have been made to use an insulating covering, made of resin or rubber, on and around the outer race of the bearing, and some of them have been put into practice.
On the other hand, the bearing hitherto suggested to minimize the problem associated with creepage is shown in FIG. 11, in which the outer peripheral surface of an outer race 51 is formed at two locations with axially spaced annular grooves for receiving therein corresponding O-rings 52. The bearing of the design shown in FIG. 11 is installed inside the journal box 54 with an oil 53 having been applied to a portion of the outer peripheral surface of the outer race 51 between the spaced O-rings 52. In the assembled condition, since the oil 53 is retained between the O-rings 52 and also between the outer peripheral surface of the outer race and the inner peripheral surface of the journal box 54, the creeping force can be reduced and, also, any possible creepage can be minimized by the effect of a frictional force brought about by the O-rings 52.
However, the insulating covering provided on the outer race of the prior art bearing is of a type that is integrally formed with the outer race by the use of an insert-molding technique. The use of the insert-molding technique renders the process of assembling the bearing to be complicated to such an extent that the production line has to be altered from that hitherto employed, resulting in increase of the cost of manufacture of the bearing.
Also, the creep-resistant bearing of the design shown in FIG. 11 has no function of resisting against the electric corrosion and, likewise, the bearing having the resin covering as described above has no function of resisting against creepage. Thus, although the bearing designed to minimize the problem associated with either the electric corrosion or the creepage is currently available in the market, no bearing has yet been developed which is effective to minimize both problems associated with the electric corrosion and the creepage. Moreover, the creep-resistant bearing shown in FIG. 11 has an additional problem in that complicated and time-consuming procedures are required to install the bearing on a machinery, and it often occurs that the worker fails to mount the O-rings 52 on the outer race of the bearing.
As a bearing effective to minimize both the electric corrosion and the creepage, the assignee of the present invention has suggested, in their Japanese Patent Application No. 6-23170, such a bearing as shown in FIG. 12. More specifically, the bearing shown in FIG. 12 includes an insulating covering 62 covering an outer peripheral surface and opposite end faces of the outer race 61, and two annular expansion compensating layers 63 of a generally ring-shaped configuration mounted on an outer peripheral surface of the insulating covering 62 in axially spaced relation to each other, each annular expansion compensating layer 63 being made of a resin having a coefficient of linear expansion greater than that of the insulating covering 62.
However, the structure employed in this suggested bearing to minimize the creepage is intended, where the journal box 64 is made of a material such as aluminum having a coefficient of linear expansion far greater than that of the bearing outer race 61, to avoid occurrence of creepage which would result from loosening of an engagement between the outer race 61 and the journal box 64 due to increase in temperature and, accordingly, the relatively great coefficient of linear expansion possessed by the expansion compensating layers 63 is utilized to avoid the creepage which would otherwise occur at an increased temperature. For this reason, such an anti-creep function as described above cannot be exhibited in the standard bearing device of a kind in which the unique relationship as to the coefficient of linear expansion cannot be obtained among the component parts of the bearing device.
Also, the expansion compensating layers 63 are so configured as to have a flat smooth outer peripheral surface to permit it to be held in tight contact with the journal box 64 to secure the anti-creep flnction assuredly and, accordingly, any axial displacement of the outer race 61 relative to the journal box can also be avoided. For this reason, the bearing of the structure shown in FIG. 12 is not suited as a bearing on a free side that permits an axial displacement of a shaft, that is inserted through a bore of the inner race, under the influence of a thermal expansion of the shaft, that is, as a bearing effective to accommodate an axial expansion of the shaft.